Finished piece cracked

[Dorrie McGregor]

I was working with a group of teens this summer, making glass masks. A simple project of taking a rectangle of Bullseye tekta, 3 mm, and a second layer on top. After we tack fused the first layer, I slumped it over a lamp bender mold. One piece (which had just a border of the second layer) cracked on me. (Looks like it happened as the kiln cooled. As this was a two layer project, I went up 250 degrees/hr; held at 1000 degrees; then up to 1250 for 10 minutes, afap to 900 degrees for an hour, then 150 degrees to 750 and then let the kiln cool at a normal rate.

The crack occurred at one corner, where two layers met one layer and is starting to travel through the piece (which is primarily 1 layer clear in the middle).


My question: now that it has been slumped over a lamp bender shape, is it possible to take it back up through the process and heal the crack?

Thanks.

[Brad Walker]

No, the crack won't heal unless it is reheated to a full fuse, and even then you will almost certainly be able to see where the break occurred. Heating to a full fuse will of course flatten the piece as well.

[Morganica]

I was working with a group of teens this summer, making glass masks. A simple project of taking a rectangle of Bullseye tekta, 3 mm, and a second layer on top. After we tack fused the first layer, I slumped it over a lamp bender mold. One piece (which had just a border of the second layer) cracked on me. (Looks like it happened as the kiln cooled. As this was a two layer project, I went up 250 degrees/hr; held at 1000 degrees; then up to 1250 for 10 minutes, afap to 900 degrees for an hour, then 150 degrees to 750 and then let the kiln cool at a normal rate.

The crack occurred at one corner, where two layers met one layer and is starting to travel through the piece (which is primarily 1 layer clear in the middle).


My question: now that it has been slumped over a lamp bender shape, is it possible to take it back up through the process and heal the crack?

Thanks.

As Brad said, nope--if you get it hot enough to re-fuse the crack you'll also flatten the piece, and unless you get it so hot that the glass softens and flows a lot (and probably causes other problems), you'll see the scar. More than likely, though, when you start heating it again the crack will finish and the piece will come apart.

I'd either take the chance that it won't come apart and fuse the piece flat, or go ahead and complete the break, then lay the pieces down on the kilnshelf and slump them flat. Either way, see what you have when it finishes, and try trimming it up and remaking. If you can add more decoration to the broken areas you might be able to disguise the scar.

But the key point in your story is here: "After we tack fused the first layer, I slumped it over a lamp bender mold. One piece (which had just a border of the second layer) cracked on me." You're doing a tack-fuse, and more than likely you simply didn't anneal long enough. If it's a two-layer tack-fuse, the schedule would call for annealing as if it were at least four layers, possibly 5 or 6, of flat fuse. Tack-fuses behave as if they're separate pieces of glass trying to pull away from each other, and they need MORE time than a flat fuse to anneal properly.

An hour anneal soak with a fast drop isn't enough.

[Dorrie McGregor]

Thanks - Great Info. So next question: given that I have seven other masks that did not break in the kiln, are they sufficiently annealed? Or should I flatten them all, take them through a four hour annealing cycle to make sure that they won't be unstable in the future? And if so, would taking them to 1250 in a slumping mold and then hold at 900 for four hours be the correct course?

Dorrie

[Morganica]

Dunno--it depends on how the masks were constructed and where/what the tack-fused decoration is. It's certainly possible that you were right on the edge of stability with all the masks and only the one with the biggest stresses cracked. And that the rest could crack if subjected to some stress in future (for example, if they were hung on a sunny wall that heated them a bit).

But it's hard to tell, especially without seeing them. It depends a lot on the tack-fuse decoration. If the masks were mostly transparent glass and you had access to a couple pieces of polarizing film (you can actually use the lenses from an old pair of polarizing sunglasses), you could check them for stress (Brad probably has something in his warmglass tutorials that would show you how to do this). The problem is, if the masks are mostly opaque glass you won't be able to tell--you need the light shining through.

There are a couple of less reliable methods. One is to see where the cracks originate on the broken piece and inspect the other masks for similar features. If the crack was caused by a tack-fused component, the crack will likely outline or appear to originate near one or more corners of the component. Look for areas with similar color and shape combinations on the other masks (be especially careful of very light opaque glass mixed with dark glass, and of a lot of small pieces lightly tacked on to a big, solid surface). Push gently on those areas, see if you can feel any kind of flex or give, see daylight between pieces, or if a light tap produces a thunk instead of a ting (indicating there might be internal cracks).

Another method is to stick the masks in the dishwasher and run them through a full cycle (without soap). Or try lightly vibrating the piece by grinding the edge, resting the edge against some kind of machinery, anything that gives the glass a little bit of stress. Dishwashing and coldworking are remarkably good at finding stress in glass, unfortunately, so if the masks survive, they're probably OK.

If you think they are stressed, you don't need to take them all the way back to slumping temps to re-anneal. The danger is that heating will exacerbate the stress, so there's a possibility that reheating the masks will cause them to break on the way up--you need to go very slowly.

Try perhaps 100dph to about 1100F, holding for a few minutes, and then re-annealing. BTW, an anneal isn't just the soak at 900/960F but also the cooldown. You need to follow the ENTIRE anneal schedule for 4-6 layers of glass. If it's Bullseye, that means your schedule is at least:
2 hrs soak
100dph to 800
180 to 700
600 to room temp

I probably would do what you suggest and hold for four hours before initiating a slightly slower downramp, just to be on the safe side.

And my primary kiln is firebrick, and cools much more slowly than 600dph after 700F, so I'd probably shut it off around 550F--it will cool slower on its own anyway. Whether you can do that depends on the cooling rate of your kiln (and a kiln's cooling will start out very fast and gradually slow down (way down for firebrick kilns) until it reaches room temp).

(And, since this is something that every glassist runs into eventually, I'm sure there will be a LOT more suggestions about this)

[Jerrwel]

Thanks - Great Info. So next question: given that I have seven other masks that did not break in the kiln, are they sufficiently annealed? Or should I flatten them all, take them through a four hour annealing cycle to make sure that they won't be unstable in the future? And if so, would taking them to 1250 in a slumping mold and then hold at 900 for four hours be the correct course?

Dorrie

See Graham Stones's book Firing Schedules for Glass http://www.warmglass.org/books/books/126-fsg from Warmglass.com for information about glass project structure and related issues (page 33) and suggested firing schedules. It's well worth the cost of the book to avoid (hopefully) spoiled projects and time spent dithering about developing a firing schedule.